The process of wet oxidation involves the addition of an oxidizing agent, typically air or oxygen, to an aqueous stream at elevated temperatures and pressures. The resultant "combustion" of organic or inorganic oxidizable materials occurs directly within the aqueous phase.
A significant development in the field of wet oxidation occurred with the issuance of U.S. Pat. No. 4,338,199, to Modell on Jul. 6, 1982. The Modell '199 patent discloses a wet oxidation process which is known as supercritical water oxidation ("SCWO"). In some implementations of the SCWO process, oxidation occurs essentially entirely at conditions which are supercritical in both temperature (&gt;374.degree. C.) and pressure (&gt;about 3,200 psi or 220 bar). Importantly, the SCWO process gives rapid and complete oxidation of virtually any organic compound in a matter of seconds.
A related process, known as supercritical temperature water oxidation ("STWO"), can provide similar oxidation effectiveness for certain feedstocks, but at a lower pressure. This process is described in U.S. Pat. No. 5,106,513, issued Apr. 21, 1992 to Hong, and utilizes temperatures in the range of six hundred degrees Celsius (600.degree. C.) and pressures between 25 bar to 220 bar.
The various processes for oxidation in an aqueous matrix are referred to collectively as hydrothermal oxidation, if carried out at temperatures between about three hundred seventy-four degrees Celsius to eight hundred degrees Celsius (374.degree. C.-800.degree. C.), and pressures between about 25 bar to 1,000 bar. A somewhat related process in which an oxidant is largely or entirely excluded from the system in order to form products which are not fully oxidized is called hydrothermal reforming. The processes of hydrothermal oxidation and hydrothermal reforming will hereinafter be jointly referred to as "hydrothermal treatment."
A common difficulty with some hydrothermal applications is precipitating of salts during processing, including salts which are normally water soluble such as sodium chloride (NaCl). The salt precipitants deposit on surfaces in the reactor and cause plugging of the reactor or other equipment. Further, the salt can cause fouling of heat transfer surfaces in the system. The build-up of salt precipitates can eventually necessitate an online or off-line cleaning of the system.
Many approaches have been tried to prevent or inhibit the salt precipitates from plugging the reactor and/or the formation of salt precipitates. Examples of prior approaches include (i) alternating reactors, (ii) a reversing flow reactor, (iii) a brine pool at the bottom of the reactor, (iv) adding inert particles to the feed material, (v) adding molten salts to the feed material, (vi) purge through a porous wall of the reactor, (vii) adding a cooler stream at the wall of the reactor, (viii) using a mechanical scraper, and/or (ix) using high velocity flow in the reactor. Unfortunately, these approaches include one or more of the following drawbacks: expensive, limited success in inhibiting salt accumulation, reduce efficiency of the system, and/or create a corrosion/materials problem.
Another common difficulty with some hydrothermal applications is the generating of reaction products which contain corrosive elements such as acids or bases. The corrosive elements damage the reactor and the system.
Many approaches have been tried to inhibit the formation of corrosive elements and/or to minimize the damage caused by the corrosive elements. Example of prior approaches include (i) neutralizing the feed material with a neutralizing agent, (ii) using a corrosion resistant liner in the reactor, (iii) using cold flow near the wall of the reactor, and/or (iv) purging through a porous wall of the reactor. Unfortunately, these approaches include one or more of the following drawbacks: expensive to manufacture and operate, limited success in minimizing damage caused by the corrosive elements, not applicable to all process streams, and/or creates salt precipitates which plug the reactor.
In light of the above, it is an object of the present invention to provide a system and method for hydrothermal treatment which continuously and reliably handles reaction medium containing or generating precipitating salts. Another object of the present invention is to provide a system and method for hydrothermal treatment of corrosive reaction medium. Still another object of the present invention is to provide a system and method for hydrothermal treatment which allow precipitating salts to be transported through the reactor without plugging. Yet another object of the present invention is to provide a system and method for accomplishing hydrothermal treatment which is easy to implement, simple to use, and relatively inexpensive to operate.